High spatial resolution particle detectors

ABSTRACT

Disclosed below are representative embodiments of methods, apparatus, and systems for detecting particles, such as radiation or charged particles. One exemplary embodiment disclosed herein is particle detector comprising an optical fiber with a first end and second end opposite the first end. The optical fiber of this embodiment further comprises a doped region at the first end and a non-doped region adjacent to the doped region. The doped region of the optical fiber is configured to scintillate upon interaction with a target particle, thereby generating one or more photons that propagate through the optical fiber and to the second end. Embodiments of the disclosed technology can be used in a variety of applications, including associated particle imaging and cold neutron scattering.

ACKNOWLEDGMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Contract No.DE-AC05-00OR22725 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD

The present application concerns particle detectors having high spatialresolution. The particle detectors can be used, for example, inassociated particle imaging or cold neutron scattering applications.

BACKGROUND

Particle detectors have a wide variety of applications and functions. Inradiation imaging systems, for example, particle detectors are oftenused to not only detect the presence of particles incident with the faceof the detector, but also to identify the spatial position on the facewhere the interaction occurred and in some cases the energy of theparticle. This spatial information can then be used as part of the imagereconstruction process. In associated particle imaging (“API”)applications, for instance, an alpha particle detector is used to “tag”an alpha particle with an associated neutron in both space and time. Toincrease the efficiency and accuracy with which associated neutrons aretagged, faster alpha particle detectors with higher spatial resolutionare desired. High spatial resolution particle detectors are alsodesirable for use in other contexts, such as cold neutron and otherneutron scattering experiments.

SUMMARY

Disclosed below are representative embodiments of methods, apparatus,and systems for detecting particles, such as radiation or chargedparticles, with high spatial resolution. The disclosed embodimentsshould not be construed as limiting in any way. Instead, the presentdisclosure is directed toward all novel and nonobvious features andaspects of the various disclosed embodiments, alone and in variouscombinations and subcombinations with one another. The disclosedmethods, apparatus, and systems are not limited to any specific aspector feature or combination thereof, nor do the disclosed embodimentsrequire that any one or more specific advantages be present or problemsbe solved.

One of the exemplary embodiments disclosed herein is a particle detectorcomprising an optical fiber having a first end and second end oppositethe first end. In this embodiment, the optical fiber comprises a dopedregion at the first end and a non-doped region adjacent to the dopedregion, the doped end region of the optical fiber being configured toscintillate upon interaction with a target particle, and to therebygenerate one or more photons that propagate through the optical fiber tothe second end. By having the scintillation region integrated into thefiber, the light output of the fiber can be increased. The targetparticle can be an alpha particle, a beta particle, another chargedparticle, a neutron, or other radiation or particle that interacts inthe first end of the fiber and produces photons. In certainimplementations, the particle detector further comprises aphotomultiplier communicatively coupled to the optical fiber, thephotomultiplier being configured to convert the one or more photons intoan electrical signal. The photomultiplier can be communicatively coupledto the optical fiber via a light guide or via a direct coupling. In someimplementations, the optical fiber is a first optical fiber and theparticle detector further comprises one or more additional opticalfibers, the one or more additional optical fibers also having dopedregions at respective first ends and non-doped regions adjacent to thedoped regions. In these implementations, the doped regions of theadditional optical fibers are also configured to scintillate uponinteraction with the target particle, and to thereby generate one ormore photons that propagate to respective second ends of the additionaloptical fibers. Further, the first optical fiber and the additionaloptical fibers are arranged to form a detection surface for detecting aspatial position at which interaction with the target particle occurs. Apixelated photomultiplier can be communicatively coupled to the firstoptical fiber and the additional optical fibers, the pixelatedphotomultiplier being configured to convert the one or more photons intoan electrical signal, the electrical signal being further indicative ofwhich one of the first optical fiber and the additional optical fibersinteracted with the target particle. In certain implementations, theparticle detector further comprises a coating deposited over the firstend of the optical fiber, the coating being configured to blocktransmission of one or more untargeted particles. The coating can bealuminum and have a thickness of 2 microns or less. In otherimplementations, however, the coating has a thickness greater than 2microns. In general, the coating can be selected to have a thicknessthat permits transmission of the target particle and for thickercoatings reflects photons generated in the scintillation region of theoptical fiber. In some implementations, the depth of the doped region atthe first end of the optical fiber is 50 microns or less. In otherimplementations, however, the depth is greater than 50 microns. Incertain implementations, the dopant of the doped region at the first endof the optical fiber is one of cerium, europium, or praseodymium, andthe target particle is an alpha particle. In such implementations, thedepth of the doped region is 20 microns or less. In someimplementations, the dopant of the doped region at the first end of theoptical fiber also contains lithium or boron if the target particle is acold neutron. The lithium or boron is preferably in the form ofisotopically enriched lithium six or boron ten in order to enhance thedetection of neutrons. In such implementations, the depth of the dopedregion can be between 20 and 100 microns. In some implementations, thedoped region includes multiple dopants, such as a first dopant thatinteracts with a target particle and produces a secondary particle(e.g., a dopant that creates an alpha particle upon interaction with aneutron, such as one of lithium or boron) and a second dopant thatinteracts with the secondary particle and produces a photon (e.g., adopant that interacts with an alpha particle or other reaction product,such as one of cerium, europium, or praseodymium).

Also among the disclosed embodiments is a method for manufacturingpartially doped optical fibers. In one example method, end portions ofone or more optical fibers are partially doped with one or more dopingagents, the one or more doping agents being selected to generate lightat the partially doped end portions when the one or more doping agentsare activated by a target particle. An annealing process is performed onat least a portion of the partially doped end portions of the one ormore optical fibers. A coating is then deposited over the end portionsof the one or more optical fibers. In certain implementations, thecoating is formed of a material (e.g., aluminum) by using physical vapordeposition, chemical deposition, or other coating methods to form acoating that blocks transmission of one or more untargeted particles.The act of partially doping the end portions can comprise implantingions into the end portions of the one or more optical fibers, andvarying an implantation energy so that the implanted ions have a depthin the end portions of the one or more optical fibers selected toscintillate with the target particle. In such implementations, the ionscan be one or more of cerium ions, europium ions, or praseodymium ions,and the depth can be selected to scintillate with an alpha particle orto produce other reaction products resulting from an interaction with aneutron. For such implementations, the depth of doping can be 20 micronsor less. In other implementations, the act of partially doping the endportions comprises diffusing ions into the end portions of the one ormore optical fibers to a depth selected to interact with the targetparticle. In such implementations, the ions can be one or more oflithium ions or boron ions, and the target particle can be a neutron.For such implementations, for alpha detectors, the depth can be 20microns or less.

Another embodiment disclosed herein is a neutron radiography systemcomprising a neutron source, one or more neutron detectors positioned todetect at least some of the neutrons generated by the neutron source, aninterrogation region located between the neutron source and the one ormore neutron detectors, and an alpha particle detector configured todetect alpha particles associated with neutrons generated by the firstneutron source. In this embodiment, the alpha particle detectorcomprises one or more alpha particle scintillation regions integrallyformed within a corresponding one or more light transmission elements.In certain implementations, the one or more light transmission elementscomprise optical fibers, and the alpha particle scintillation regionscomprise doped end portions of the optical fibers. In someimplementations, a majority of the bodies of the optical fibers areundoped. In certain implementations, the doped end portions are dopedwith one or more of cerium, europium, or praseodymium. In someimplementations, the alpha particle detector has a spatial resolution of100 microns or less. In certain implementations, the alpha particledetector has a timing resolution of 2 nanoseconds or less. In someimplementations, the alpha particle detector further comprises a coatingconfigured to prevent transmission of light and scattered acceleratedparticles in the generator to the scintillation regions. In certainimplementations, the alpha particle detector does not include ascintillating element separable from the one or more light transmissionelements. In some implementations, the system further comprises apixelated photomultiplier communicatively coupled to the lighttransmission elements of the alpha particle detector and configured toconvert light from the one or more light transmission elements intoelectrical signals, the electrical signals being indicative of which oneof the one or more light transmission elements was activated by a targetparticle. In such implementations, the pixelated photomultiplier can becommunicatively coupled to the light transmission elements through alight guide or a direct coupling.

A further embodiment disclosed herein is a neutron scattering systemcomprising a neutron source, a scattering target positioned in a pathalong which neutrons from the neutron source travel, and one or moreneutron detectors positioned to detect one or more scattered neutronsscattered by the scattering target. In this embodiment, the one or moreneutron detectors comprise light transmission elements having partiallydoped end portions, and the partially doped end portions comprise one ormore dopants selected to interact with an incident scattered neutron andthereby generate a photon. In certain implementations, the one or moredopants comprise a first dopant selected to generate a secondaryparticle upon interaction with the incident neutron, and a second dopantselected to generate a photon upon interaction with the secondaryparticle. For instance, the first dopant can be one or more of lithium-6or boron-10, and the second dopant can be one or more of cerium,europium, or praseodymium. In some implementations, the majority of thebodies of the light transmission elements are undoped. In certainimplementations, the partially doped end portions of the lighttransmission elements are covered by a coating (e.g., aluminum) thatpermits transmission of neutrons and reflects photons generated withinthe bodies of the light transmission elements.

The foregoing and other objects, features, and advantages of embodimentsof the invention will become more apparent from the following detaileddescription, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an associated particle neutronradiography (“APNR”) system using an embodiment of the disclosedtechnology as an alpha particle detector.

FIG. 2 is a schematic block diagram of a second APNR system that alsouses an embodiment of the disclosed technology as an alpha particledetector. In FIG. 2, the APNR system is illustrated as interrogating ametal pipe concealing a puck-shaped plastic object with a central hole.

FIG. 3 is a time distribution of counts (after an alpha detector count)at a neutron detector from the APNR system of FIG. 2.

FIG. 4 shows the attenuation length as a function of lateral dimensionfrom a row of neutron detectors of the APNR system of FIG. 2 at a heightof the neutron detectors that is above the top of the puck-shapedplastic object within the metal pipe.

FIG. 5 is a slice through a reconstructed three-dimensional image of theobject shown in FIG. 2. The image in FIG. 5 is generated from multipleprojection images.

FIG. 6 is a schematic block diagram of a first exemplary particledetection system using an embodiment of the disclosed technology as afiber optic face plate.

FIG. 7 is a schematic block diagram of a second exemplary particledetection system using another embodiment of the disclosed technology asa fiber optic face plate.

FIG. 8 is a flowchart illustrating a method of manufacturing partiallydoped optical fibers according to embodiments of the disclosedtechnology.

FIG. 9 is a schematic block diagram of a cold neutron scattering systemusing an embodiment of the disclosed technology as a radiation detector.

DETAILED DESCRIPTION I. General Considerations

Disclosed below are representative embodiments of methods, apparatus,and systems for detecting particles, such as radiation or chargedparticles. The disclosed methods, apparatus, and systems should not beconstrued as limiting in any way. Instead, the present disclosure isdirected toward all novel and nonobvious features and aspects of thevarious disclosed embodiments, alone and in various combinations andsubcombinations with one another. Furthermore, any features or aspectsof the disclosed embodiments can be used in various combinations andsubcombinations with one another. The disclosed methods, apparatus, andsystems are not limited to any specific aspect or feature or combinationthereof, nor do the disclosed embodiments require that any one or morespecific advantages be present or problems be solved.

Although the operations of some of the disclosed methods are describedin a particular, sequential order for convenient presentation, it shouldbe understood that this manner of description encompasses rearrangement,unless a particular ordering is required by specific language set forthbelow. For example, operations described sequentially may in some casesbe rearranged or performed concurrently. Moreover, for the sake ofsimplicity, the attached figures may not show the various ways in whichthe disclosed methods, apparatus, and systems can be used in conjunctionwith other methods, apparatus, and systems.

II. Associated Particle Imaging

Exemplary embodiments of the disclosed particle detectors areparticularly well suited for use as an alpha particle detector in anassociated particle imaging (“API”) system. Accordingly, this sectionprovides a description of API and exemplary API systems in which thedisclosed particle detectors can be used.

In general, API involves “tagging” a neutron emission in time,direction, or both time and direction by detecting a particle that isassociated with the creation of a neutron. For example, embodiments ofthe disclosed technology use a deuterium-tritium generator as a neutronsource. The deuterium-tritium generator produces monoenergetic neutrons(sometimes designated herein as “n” particles) and alpha particles(sometimes designated herein as “⁴He” or “α” particles) that travel innearly opposite directions from one another. By detecting the arrival ofan alpha particle and its position (e.g., in two dimensions) at an alphaparticle detector located in a known geometry from the neutron source,the time and/or direction of the neutron emission can be determined.Although the direction of travel of the neutron is not exactly oppositeof its associated alpha particle, the direction of travel is fixed andcan be predicted accurately after accounting for the momentum of theparticles in the deuterium beam. Accordingly, the direction of travel ofthe neutron can be determined accurately from the detected position ofits associated alpha particle. In this way, the alpha particle can beused to “tag” the neutron emission. Further, because the time-of-flightof the neutron is fixed in a system that has a known geometry and thatproduces monoenergetic neutrons, a neutron detected at an array ofneutron detectors positioned distally from the neutron source can bepositively identified as the “tagged” neutron if it arrives in theexpected time window and at an expected position at the detectors.

Furthermore and as more fully explained below, embodiments of thedisclosed technology use a transmission imaging approach. In particular,images of an interrogated object are generated based on the number ofdetected neutrons that are transmitted through the interrogated objectwithout scattering or causing fission with nuclei in the interrogatedobject. The resulting images can be generated, for example, bynormalizing the detected counts in the neutron detector to thoseproduced by the system when no object is present in the system.

The combination of API with transmission imaging is sometimes referredto herein as associated particle neutron radiography (“APNR”). The useof time and direction tagging allows embodiments of the APNR system toeffectively remove measurement noise resulting from scattered neutrons(this technique is sometimes referred to as “electronic collimation”).The elimination of scattered neutrons enables high-contrast images to begenerated, even through thick objects (e.g., large cargo containers),without the need for any physical collimation or shaping of the neutronbeam. Thus, embodiments of the APNR system can be free of a physicalcollimator. The elimination of the need for physical collimation alsoenables wide cone-beam imaging without compromising image contrast. Withwide cone-beam imaging, two-dimensional arrays of neutron detectors canbe used, thus allowing the system to detect and use many more neutronsduring the imaging process than is possible with conventional fan-basedimaging. This ability to collect data in two dimensions also compensatesfor any loss in imaging capability that results from using an associatedparticle imaging technique. Additionally, wide cone-beam imaging and theabsence of a physical collimator also enables the neutron source to bepositioned close to the interrogated object, resulting in a compactgeometry that requires less shielding overall. The overall footprint forembodiments of the APNR system can therefore be much smaller and lighterthan conventional systems. Furthermore, APNR allows multiple neutronsources to be used (e.g., to be used simultaneously) during the neutroninterrogation and image generation process. Consequently, multipleprojection images from different angles can be generated simultaneously,significantly accelerating the image capture and reconstruction process(e.g., using computed tomography techniques, such as a filtered backprojection technique).

FIG. 1 is a schematic block diagram of an exemplary APNR configuration100 in which an embodiment of the disclosed technology is used as analpha particle detector. In general, the configuration 100 illustratesthe basic components used to perform APNR using a single neutron source.Additional neutron sources can also be used.

In FIG. 1, a neutron source 110 generates neutrons that interrogate anobject 150 and that are detected by an array of neutron detectors 140.In the illustrated embodiment, the neutron source 110 is a portabledeuterium-tritium (“DT”) generator. In other embodiments, however, otherneutron sources are used (e.g., other monoenergetic neutron generators,such as a deuterium-deuterium generator, or a non-monoenergeticCalifornium neutron source, or any other neutron source). The DTgenerator illustrated in FIG. 1 includes a deuterium accelerator 112that produces a deuterium beam that strikes a tritium-impregnated target114 at a fixed location in the generator (sometimes referred to as the“neutron production spot”). In other embodiments, however, a tritiumbeam can strike a deuterium-impregnated target, or a mixeddeuterium-tritium beam can strike a mixed deuterium-tritium target. Theresulting interaction in the illustrated embodiment produces amonoenergetic neutron (e.g., a 14.1 MeV neutron) and an alpha particle(e.g., a 3.5 MeV alpha particle):d+t→n+ ⁴He.  (1)In the illustrated configuration, the deuterium accelerator 112 and thetritium-impregnated target 114 are located in fixed positions inside asealed tube 116. With this configuration, the source of the neutrons canbe considered to be the location of the tritium-impregnated target 114that interacts with the deuterium beam. In certain embodiments, forinstance, the neutron production spot size of the target (and thus thesize of the source) is reduced or minimized so that the paths of thegenerated neutrons can be determined with higher accuracy. Also, becauseof the penetrating power of neutrons, the sealed tube 116 need not haveany physical window or other aperture for emitting the neutrons. In theillustrated embodiment, the total neutron emissions from the neutronsource 110 are roughly isotropic.

The configuration 100 further includes an alpha particle detector 120.In the illustrated embodiments, the alpha particle detector 120comprises a fiber optic face plate 122 formed from multiple fibers thathave been partially doped to create a scintillation region 124 at oneend of the fibers inside the generator. In this way, the fiber opticplate itself includes a scintillation material. Furthermore, theillustrated fiber optic face plate 122 is coupled to a fast,position-sensitive photomultiplier tube 126 configured to detectactivation of one or more of the scintillation fibers in the fiber opticplate and generate a corresponding electrical signal representative ofthe location on the scintillation surface of the fiber optic face plate122 where activation occurred. In the illustrated embodiment, the fiberoptic face plate 122 is located internally within a sealed tube 190 ofthe APNR configuration 100, whereas the photomultiplier tube 126 islocated externally. Thus, the two components are separated by a smallgap, which can be filled with an optical coupling jelly. In otherembodiments, however, both the fiber optic face plate 122 and thephotomultiplier tube 126 are located internally within the tube 190.Furthermore, and as more fully explained below, a light guide or anextended bundle of fiber optic wires can be used to couple the fiberoptic face plate 122 to the photomultiplier tube 126. Embodiments of thefiber optic face plate operate with a high timing resolution (e.g., 10ns or less, 3 ns or less, or 1 ns or less) and thus improve the abilityof the APNR configuration to discriminate between neutrons generated atdifferent times. Embodiments of the fiber optic face plate also operatewith high spatial resolution (e.g., 100 microns or less) and thusimprove the precision with which an alpha particle path and neutron pathcan be determined. Additional details of exemplary embodiments of thefiber optic face plate 122 are described below with respect to FIGS. 6and 7.

The configuration 100 additionally comprises an array of neutrondetectors 140 positioned distally from the neutron source 110. In theillustrated embodiment, the neutron detectors compriseposition-sensitive two-dimensional neutron “block detectors.” Eachneutron block detector can comprise any number of detectors in anyarrangement (e.g., 10×10 arrays). The illustrated detectors are coupledto a plurality of photomultiplier tubes whose shared response can beused to determine the position on the array where the neutroninteraction actually occurred. In other embodiments, other suitableneutron detectors are used (e.g., plastic scintillators, proton recoilscintillators, or other such fast neutron detectors). Furthermore, thetiming resolution of the detectors is desirably high (e.g., a rise anddecay time of 10 ns or less, 3 ns or less, or 1 ns or less) in order toimprove the ability of the neutron detector to discriminate betweenneutrons generated at different times. The illustrated array of neutrondetectors 140 can be used to determine the position of the detectedneutron relative to the neutron source 120, as well as the time offlight of a neutron emitted from the neutron source 110. Aninterrogation region 160 in which an interrogated object 150 ispositioned is located between the array of neutron detectors 140 and theneutron source 110.

The array of neutron detectors 140 and the alpha-particle detector 120also help define the shape and size of the neutron beam emitted from theneutron source 110 that is used for neutron imaging. Although neutronsare emitted from the neutron source roughly isotropically, only afraction of the emitted neutrons are time and directionally tagged andare useful for imaging purposes. In particular, the neutrons that areuseful for imaging purposes comprise those neutrons that can be detectedby the neutron detector 140 and whose associated alpha particles can bedetected by the alpha-particle detector 120. The three-dimensional spacetraversed by these neutrons is referred to herein as the neutron beam,and typically forms a cone beam (e.g., cone beam 180) since the shape ofthe alpha-particle detector 120 is usually circular. Other beam shapesare possible, however, depending on the particular shape andconfiguration of the alpha-particle detector 120. If the alpha detectoris subdivided into pixels, the total neutron beam is subdivided into anarray of neutron beamlets of any desired shape depending on thesubdivision of the alpha detector.

To illustrate the principles of the APNR method, FIG. 1 furtherillustrates two possible neutron paths. A first neutron path 130 travelsthrough the interrogated object 150 and is detected at position 142 onthe face of the array of neutron detectors 140. A second neutron path132 also travels through the interrogated object 150 but is scattered orotherwise interacts in the object 150. The resulting scattered orfission neutron arrives at a position 144 on the array of neutrondetectors 140. Because the scattered or fission neutron does not arriveat the correct time and/or position on the neutron detector, thescattered or fission neutron detected at position 144 can be ignored forpurposes of creating a projection image of the interrogated object 150.

The configuration 100 further comprises an image processing system 170coupled to the neutron detector 140 and the alpha particle detector 120.In certain embodiments, the image processing system 170 comprises acomputer-based system (e.g., system comprising a computer processor,non-transitory memory, and non-transitory storage media) that executesimage processing software. The image processing software can comprise,for example, computer-executable instructions stored on one or morenon-transitory computer-readable media (e.g., volatile memory,non-volatile memory, or magnetic storage devices, such as hard drives)which when executed by a computer cause the computer to perform an imageprocessing method (e.g., any of the image processing methods disclosedbelow).

FIGS. 2-6 illustrate an exemplary image processing method that can beperformed by the image processing system 170. FIGS. 2-6 also showexemplary images that can be obtained as a result of using embodimentsof the disclosed technology. For illustrative purposes, the exemplaryimage processing method is described in relation to the configurationshown in FIG. 2.

FIG. 2 shows an APNR configuration 200 comprising a neutron generator210, which is a sealed tube DT generator that has an associated alphaparticle detector comprising an embodiment of the fiber optic face platedisclosed below with respect to FIGS. 6 and 7. The APNR configurationfurther comprises a two-dimensional array of neutron detectors 220configured to have a semi-spherical shape. An interrogation region 230is defined between the neutron generator 210 and the array of neutrondetectors 210. The semi-spherical shape of the neutron detector array220 allows each individual neutron detector of the array 220 to beequidistant from the neutron source in the neutron generator 210. Inother embodiments, the neutron detector array 220 is flat, partiallyspherical, fan-shaped, or has some other shape. For example purposes,the objects being interrogated in the interrogation region 230 are ametal pipe 240 that conceals a puck-shaped plastic object 242 with acentral hole.

To implement the exemplary image processing method, certain baselinemeasurements and images can be determined. For example, in order toestablish the expected time-of-flight of the neutrons emitted from theneutron source, counts at each of the neutron detectors on the array ofneutron detectors 220 can be captured with respect to the time fromdetection of the associated alpha particle at the corresponding alphadetector pixel (e.g., the alpha detector pixel that indicates neutronemission in the direction of a particular neutron detector).

FIG. 3 is a representation 300 showing a plot 302 from the results fromone such neutron detector. For the neutron detector associated withimage 300, the plot 302 shows that the time-of-flight for a neutron isbetween about 15-20 nanoseconds from the time the associated alphaparticle is detected at the corresponding alpha detector position. Thisinformation can be used to select the proper time period or time window(e.g., time window 310) in which to count a detected neutron as beingthe “tagged” neutron associated with a corresponding alpha particledetected at the alpha particle detector. The time window can comprisemultiple time bins (or sampling periods) at which the neutron detectorsoperate. Further, the time window can have a variety of lengths relativeto the observed peak. For any given interrogation period, the countsthat are observed as falling within the selected time window with andwithout the object in place are collected and used to generate aprojection image.

The exemplary imaging process also uses a normalization image I₀. Inparticular embodiments, the normalization image I₀ is the image acrossone or more (e.g., all) of the neutron detectors of the neutron detectorarray 220 when no object is present in the interrogation region. Theimage I₀ can comprise, for example, the count rate of neutrons at eachneutron detector across the neutron detector array 220 during theappropriate time window for tagged neutrons.

A projection image can then be taken with the object in theinterrogation region 230. The image taken with the object in theinterrogation region 230 results in a signal I for a given neutrondetector in the array of neutron detectors:I=I₀e^(−μL)  (2)where I₀ is the normalization image for the given neutron detector, μ isthe attenuation coefficient for the object being interrogated, and L isthe path length through the object.

If there are n multiple materials between the neutron source and theneutron detector, then the projection image observed is known to be asummation in the exponent:I=I₀e^(−Σ) ^(i=0) ^(n) ^(μ) ^(i) ^(L) ^(i)   (3)Consequently, the projection image of the object I can be normalizedusing the normalization image I₀, resulting in the summation ofattenuations:

$\begin{matrix}{{\log\left( \frac{I}{I_{0}} \right)} = {- {\sum\limits_{i = 0}^{n}\;{\mu_{i}L_{i}}}}} & (4)\end{matrix}$The attenuation lengths for each neutron detector in the neutrondetector array can then be plotted together in order to form atwo-dimensional projection image of the interrogated object. Forexample, FIG. 4 is an image 400 showing a plot 402 of attenuationlengths computed from a projection image of the lead pipe 240 at aheight of neutron detectors above the top of the plastic object 242 inthe neutron detector array 220 of the APNR configuration 200.

If additional neutron sources are present or if the object is rotated orotherwise moved (or if the neutron generator and the array of neutrondetectors are rotated or otherwise moved) to a new position,normalization images and projection images of the object can be taken atmany different orientations around the object. From the resultingprojection images, three-dimensional reconstructions can be performed toarrive at a complete three-dimensional image or representation of theinterrogated object. For example, in particular embodiments, a filteredback projection technique is used to construct a three-dimensional imagefrom the projection images from the multiple neutron sources as well asthe images from different orientations of the multiple neutron sources.In other embodiments, maximum likelihood estimation maximization(“MLEM”) techniques, ordered subset estimation maximization (“OSEM”)techniques, or other iterative reconstruction techniques are used. Thethree-dimensional image or representation of the interrogated object canbe displayed to a user of the system (e.g., on a suitable displaydevice) and/or stored on computer-readable media (e.g., non-transitorycomputer-readable media).

FIG. 5 is an image 500 of a slice through a three-dimensionalreconstruction assembled from multiple projection images. In particular,the image 500 is a slice through the metal pipe 240 and the plasticobject 242. The image 500 is a slice that is generated from the planarimages obtained from interrogating the sides of the objects using anappropriate three-dimensional reconstruction technique. The image 500clearly shows the lead pipe at region 510 and the plastic object atregion 512. Image 500 also shows that the lead pipe and the plasticobject can be differentiated from one another as a result of the neutronimaging.

III. Exemplary Embodiments of Particle Detectors Having Partially DopedScintillation Fibers

FIG. 6 is a schematic block diagram showing a first embodiment of adetection system 600 according to the disclosed technology. Thedetection system 600 can be used with embodiments of the APNR systemdiscussed above, as well as with other system, as more fully disclosedbelow. The illustrated detection system 600 comprises a fiber optic faceplate 610, a light guide 620, and a photomultiplier 630. In certainembodiments, the light guide 620 is not present. Furthermore, the fiberoptic face plate 610, the light guide 620, and the photomultiplier 630are shown in FIG. 6 as having a generally cylindrical shape, althoughthey could have any other shape as well (e.g., square, rectangular,partially spherical or elliptical, or other such shape). In theillustrated configuration, the fiber optic face plate 610, the lightguide 620, and the photomultiplier 630 are separated from one another bytwo small gaps 660, 662, which can be filled with optical coupling jellyor which represent a wall or other boundary (e.g., a wall of a sealed DTgenerator tube). In some embodiments, one or both of the illustratedgaps 660, 662 are absent.

The fiber optic face plate 610 of the illustrated embodiment is formedfrom two or more optical fibers bundled together as shown in thepartially cut-away region of FIG. 6. A representative optical fiber isshown as optical fiber 612. Although the optical fibers shown in FIG. 6have a generally cylindrical shape, they could have any other shape(e.g., square, rectangular, polygonal, ellipsoidal, or any other suchgeometry). Additionally, the optical fibers can be arranged in a widevariety of arrangements that are appropriate for spatial detection(e.g., a two-dimensional array, spherical arrangement, and other sucharrangement). The diameters of the two or more optical fibers can alsovary, but in certain embodiments are 100 microns or less. The opticalfibers are typically fibers formed of fused silica, but can be formedfrom other glass forming materials as well (e.g., fluorides, phosphates,germanates, or plastics). In some embodiments, for example, the opticalfibers are also formed from single crystal fibers, such as yttriumaluminum perovskite (“YAP”) fibers, yttrium aluminum garnet (“YAG”)fibers, or aluminum oxide (Al₂O₃) fibers. In certain embodiments, theoptical fibers are fused together to form a bundle using a bonding glasswith a matching coefficient of thermal expansion and appropriaterefractive index to preserve the light guiding properties of the fibers.

In the illustrated embodiment, one or more of the optical fibers arepartially doped with one or more dopants selected to scintillate whenincident with a neutron, charged particle, or other activating particle.For example, the representative optical fiber 612 includes a dopedregion 613. In the illustrated embodiment, the remaining length of theoptical fibers is not doped with any selected dopant (e.g., the majoritylength of the optical fibers is undoped). The undoped length produces nolight from particle interactions.

In some implementations, multiple dopants are used to create the dopedregion. For example, a first dopant in the doped region can interactwith the target particle and thereby produce a secondary particle, and asecond dopant in the doped region can interact with the secondaryparticle and create one or more photons through a scintillation process.In one exemplary implementation, for instance, the ends of the fiber aredoped with ⁶Li, which produces alpha particles and tritons from slowneutrons, and with Ce³⁺, which produces photons from interactions withalpha particles. The number, type, and depth of the dopants used willvary depending on the target particle. Further, the examples describedherein should not be construed as limiting, as additional or alternativedopants can be used to create other cascades of interactions to producephotons within the optical fibers from a target particle (e.g., a betaparticle or other charged particle). Additionally, in embodiments withmultiple dopants, any two or more of the dopants can be in overlappingdoped regions, partially overlapping doped regions (e.g., with the dopedregion for producing photons (or, alternatively, for interacting withthe target particle) extending deeper than other doped regions), ornon-overlapping doped regions (e.g., with the doped region for producingphotons (or, alternatively, for interacting with the target particle)being located deeper than other doped regions).

In general, the one or more dopants and the depth of doping can beselected in view of the intended function and activating particle forwhich the optical fibers are targeted. In certain embodiments (such asthe illustrated embodiment in which the optical fibers will be used asalpha particle detectors (e.g., in API applications)), the dopant iscerium (Ce). In specific implementations, the dopant is Ce³⁺. In otherembodiments, the dopant is one or more of europium (e.g., Eu²⁺) orpraseodymium (e.g., Pr³⁺). In some embodiments (such as embodiments inwhich the optical fibers will be used as neutron particle detectors),the second dopant is lithium (e.g., ⁶Li) or boron (¹⁰B). As noted above,the doped regions of the optical fibers can be further doped orotherwise combined or enriched with other materials that are selected toenhance the detection of the targeted activating particle. For instance,the doped region can be further doped with a dopant selected to producephotons from a secondary particle generated by a first dopant. Forexample, a region doped with lithium or boron to produce alpha particlesfrom incident neutrons can be further doped with cerium, europium, orpraseodymium to produce photons from the alpha particles.

In certain embodiments, both the optical fibers that form the bundle andthe doping agent are inorganic. This allows the resulting fiber opticface plate to withstand high temperatures without losing itsfunctionality. For instance, when sealed tube DT generators aremanufactured, the tube and its components are baked to over 300° C. toreduce impurity content.

The depth to which the one or more optical fibers are doped can varyfrom implementation to implementation. In particular, the depth ofdoping will vary depending on the particle and the particle energy forwhich the optical fibers are targeted. Although the depth of doping canbe of any size, in certain embodiments, the doped region has a depth (orlength) of 1-100 microns. For optical fibers designed to scintillatewith incident alpha particles, the depth of doping can be between 1-20microns, and in certain desirable embodiments is between 5-10 microns.

A coating 640 is also shown in FIG. 6 and coats the scintillation face650 of the fiber optic face plate 610. The coating 640 shown in FIG. 6coats the entire scintillation face 650 of the fiber optic face plate610. In other embodiments, however, the coating 640 only coats a portionof the face. For instance, in certain embodiments, the coating isapplied so that it coats the individual ends of the optical fibers. Ingeneral, the coating 640 is configured to shield or prevent transmissionof certain selected particle to the scintillation ends of the opticalfibers. In certain embodiments, the coating is a coating of aluminum (orother suitable material) and prevents light or scattered tritons ordeuterons from reaching the scintillation ends of the fibers. Thecoating can have a variety of thicknesses, but in certain embodiments is1-5 microns, and in particular embodiments is 1.5 microns or less (e.g.,1 micron). The thickness of the coating can be varied so that thecoating not only achieves the described shielding effects, but alsooperates to reflect photons generated within the optical fibers, therebyincreasing the effective light output of the fibers.

In the embodiment illustrated in FIG. 6, the photomultiplier 630 iscoupled to the non-scintillating face 652 (or end) of the fiber opticface plate 610 via a light guide 620. The photomultiplier 630 can be anyposition-sensitive photomultiplier. In some embodiments, thephotomultiplier 630 has pixelated regions or otherlight-to-electric-signal conversion regions that have a size andorientation corresponding to the size and orientation of the opticalfibers in the fiber optic face plate. In other embodiments, however, alight guide (such as light guide 620) or other light transmission system(such as the bundle of optical fibers shown in FIG. 7) is used to directlight from the non-scintillating face 652 of the fiber optic face plate610 to corresponding pixelated regions or conversion regions of thephotomultiplier 630. Examples of suitable photomultipliers include thephotomultiplier tube assemblies available from Hamamatsu Photonics(e.g., the Hamamatsu 9500). Although a photomultiplier 630 is shown inFIG. 6, any light-to-electric-signal conversion system could be used.

As noted, embodiments of the detection system 600 can be used in an APNRsystem such as shown in FIG. 1. In such a system, or in any system inwhich the detection system 600 is used to detect alpha particles, thesystem can operate follows. An alpha particle passes through the coating640 that covers the scintillation face 650 (or first end) of the opticalfibers inside the vacuum-sealed DT generator. As noted, the coating 640can be an aluminum layer (e.g., having a thickness of 5 microns or less)and can prevent light and scattered deuterium and tritium from thegenerator from reaching the scintillation regions of the optical fibers.The alpha particle stops in the scintillation region of one of thefibers and produces light (one or more photons). As noted, thescintillation region can be 5 to 10 microns long. In this example, thescintillation region is thin to minimize detection of X-rays that wouldotherwise produce low amplitude background counts. The thin scintillatoris also relatively insensitive to gamma rays produced by neutrons fromthe generator and from background radiation. The light propagatesthrough the fiber to the non-scintillator face 652 (or end) of the fiberoptic face plate 610. The light coming through the non-scintillator face652 of the fiber optic face plate 610 is guided by the light guide 620to the center of a pixel of the photomultiplier 630. The light theninteracts with the photomultiplier 630, producing an electronic pulsethat defines the time and location of the alpha detection, as describedabove. As noted, the light guide is not present in some embodiments.

FIG. 7 is a schematic block diagram showing a second embodiment of adetection system 700 according to the disclosed technology. Thedetection system 700 comprises a fiber optic face plate 710 directlycoupled to a photomultiplier 730. In this embodiment, the optical fibersof the fiber optic face plate extend out of the housing of the fiberoptic face plate 710 and form a bundle of optical fibers 720. The bundleof optical fibers is routed directly to the input regions of theposition-sensitive photomultiplier 730. As illustrated, thephotomultiplier 730 generates electrical signals based on the lightreceived. In an API application, the photomultiplier 730 can be coupledto an image processing system (e.g., any of the image processing systemsdescribed above), which performs a suitable image reconstructiontechnique. A wide variety of photomultipliers can be used as thephotomultiplier 730. Suitable examples of such photomultipliers includethe photomultiplier tube assemblies available from Hamamatsu (e.g., theH9500).

In both FIG. 6 and FIG. 7, the scintillating region is integrally formedwith the light transmitting optical fibers of the faceplate. Thus, noseparate scintillation device or plate is needed to perform particledetection. This integrated design provides a number of possibleadvantages, any one or more of which can be realized in embodiments ofthe disclosed technology.

For example, one possible advantage that can be realized in embodimentsof the design illustrated in FIG. 6 or FIG. 7 is that the scintillationregion exhibits reduced light dispersion. This reduction in lightdispersion results from the integrated design of the optical fibers aswell as the coating that can be applied to the scintillation end of thefibers. In general, the light output of the optical fibers increases asthe thickness of the coating, which operates to reflect light at thefirst end of the optical fibers, increases. By contrast, in designs inwhich a separate scintillator is mounted to a fiber optic face plate, agap exists between the separate scintillator and the optical fibers.Significant light loss occurs at this gap. In addition, the separatescintillator has a finite thickness through which light disperses in alldirections. This creates an additional loss of light.

A further advantage that can be realized in embodiments of the designillustrated in FIG. 6 is that the scintillating region is integratedwith (and in some embodiments, within) individual optical fibers. Thus,when scintillation occurs at a scintillation region as a result of anincident activation particle, the resulting light is concentrated withinthe single optical fiber whose scintillation region was excited. Thisconcentration of light to a single optical fiber allows the bundle ofoptical fibers to have high spatial resolution, since the location ofthe incident particle can be identified and associated with anindividual optical fiber. In certain embodiments, then, the possibleresolution of the fiber optic face plate is the size or diameter of theoptical fibers (e.g., 100 microns or less). By contrast, in designs inwhich a separate scintillator is mounted to a fiber optic face plate,the dispersion of light in the body of the separate scintillatortypically causes the light to be incident on multiple optical fibers ofthe face plate. Thus, the spatial resolution of such a face plate isgreatly improved.

A further advantage that can be realized in embodiments of the designillustrated in FIG. 6 or FIG. 7 is that as a result of the decreasedlight loss and the increased resolution, the number of false targetparticle detections (e.g., false alpha detections) can be reduced whilethe number of real target particle detections (e.g., real alphadetections) can be increased.

A further advantage that can be realized in embodiments of the designillustrated in FIG. 6 or FIG. 7 is that the scintillating region can bedesigned to more precisely target an activating element. For instance,the depth of the doping of the optical fibers can be controlled withhigh precision, resulting in scintillation regions that have a muchhigher alpha detection rate and lower false alpha detection rate thanconventional scintillators. Such scintillation regions are also lesssensitive to background radiation or other noise (e.g., X-rays or gammarays) because they are so thin. The coating (e.g., coating 640) that isincluded in some embodiments of the disclosed technology can alsoenhance the sensitivity to real target particle detections and reducesensitivity to false target particles, background radiation, and/orother noise.

A further advantage that can be realized in embodiments of the designillustrated in FIG. 6 or FIG. 7 is the speed with which the fiber opticface plate comprising partially doped optical fibers can operate. Onaccount of the integrated design, embodiments of the disclosed fiberoptic face plate can operate with a time resolution of 10 nanoseconds orless, such as 2 nanoseconds or less or 1 nanosecond or less. Thus, whenembodiments of the disclosed technology are used in API applications, atime of flight event can be resolved to less than 2 nanoseconds, orwithin about 1 nanosecond.

A further advantage that can be realized in embodiments of the designillustrated in FIG. 6 or FIG. 7 is that the integration of thescintillating regions with the optical fibers creates a simpler systemoverall. As a result, the scintillating fiber optic face plate isdurable and less susceptible to failure or misalignment such as mayoccur in systems in which a separate scintillator is mechanicallyaffixed to a face place. This feature can be particularly advantageouswhen the fiber optic face plate is used in sealed environments, whererepairs are time-consuming, costly, and potentially not feasible.

In the embodiments illustrated in FIG. 6 and FIG. 7 and discussed above,the spatial resolution is generally defined by the size of the fibersand how many are bundled together and directed to the sensitive area ofthe pixels of the photomultiplier. For example, certain embodiments ofthe disclosed technology have a spatial resolution of 100 microns orless. When embodiments of the disclosed fiber optic face plate are usedin API systems (such as the APNR system of FIG. 1), however, the spatialresolution of the fiber optic face plate may be limited by other factorsin the system. For example, the neutron cone size corresponding to agiven alpha pixel depends also on the neutron production spot size inthe DT generator. Reducing the pixel size in the alpha detector beyondsome limit is typically not beneficial unless the neutron productionspot size is reduced also. However, there is typically a limit to theamount the neutron spot size of the DT generator can be reduced due toheating of the target. Target cooling can be used to allow more beamenergy to be incident on the target without damage.

FIG. 8 is a flowchart 800 illustrating an exemplary method formanufacturing a fiber optic face plate, such as the fiber optic faceplate illustrated in FIG. 6 or FIG. 7. The illustrated method should notbe construed as limited, however, as other manufacturing techniques canbe used to create embodiments of the disclosed technology. Additionally,any one or more of the illustrated acts can be performed alone or invarious combinations and subcombinations with one another or with othermethod acts.

At 810, a portion of one or more optical fibers are doped. For instance,in certain embodiments, less than a majority of the optical fiber isdoped, and in some embodiments, doping is performed on a short endportion of the optical fibers (e.g., an end portion of the opticalfibers that is 50 microns or less, 20 microns or less, or 10 microns orless) with a remainder of the optical fibers being undoped. The dopingcan be performed using a variety of techniques, but in certainembodiments is performed by high-energy ion implantation. Morespecifically, high-energy ion implantation can be used to implantactivator ions to form a scintillation region on the end of the opticalfibers. The activator ions can include one or more of Ce³⁺, Eu²⁺, Pr³⁺,or other trivalent ions. Other additional or alternative activator ionscan also be used depending on the particle for which the optical fibersare to be functionalized. In embodiments in which multiple dopants areimplanted, the dopants can be applied in consecutive order (e.g., thedopant selected to generate photons from a secondary particle can beimplanted first, followed by the dopant selected to interact with thetarget particle and thereby generate the secondary particle). Ingeneral, the goal of the implantation process is to form a scintillationregion on the optical fiber whose depth corresponds to the range of theincident particles (e.g., incident alpha particles). The depth of theion implantation region, and thus the size of the scintillation region,can be controlled by the implantation energy. In other words, by varyingthe implantation energy of the implantation accelerator, variousimplantation depths can be produced. If single crystal optical fibersare used to form the bundle of optical fibers (e.g., Al₂O₃ fibers,yttrium aluminum perovskite (“YAP”) fibers, yttrium aluminum garnet(“YAG”) fibers, or other such single crystal optical fibers), ionimplantation along a “channeling” direction can be used to furtherincrease the maximum depth of the ion implantation. Also, before ionimplantation is performed, the ends of the optical fibers can bepolished. A controlled diffusion process (e.g., controlled thermaldiffusion) can be used as an alternative to ion implantation insituations whether deeper doping depths are desired (e.g., depths ofgreater than 10 microns).

At 820, an annealing process is performed on the one or more dopedoptical fibers. The annealing process can be performed after or duringthe doping of the optical fibers. The annealing process is performed touniformly distribute the doping atoms. For instance, a controlleddiffusion process can be performed. In some instances, annealing is usedto increase the depth of implantation (e.g., to create a doping regionhaving a size greater than 50 microns).

At 830, a coating is applied to the doped ends of the optical fibers.For example, the coating can be a metal or other element selected toprevent passage of certain particles (e.g., gamma rays, X-rays, tritons,deuterons, or other undesired particles, depending on the application).In particular implementations in which the doped optical fibers are usedfor alpha particle detection (e.g., as part of an API system), theoptical fibers can be coated with aluminum to prevent light or scatteredtritons or deuterons from reaching the scintillation end of the opticalfibers. This coating may be applied using physical vapor deposition,chemical deposition, or other standard coating methods. Although thethickness of the coating will vary from implementation toimplementation, a coating of 5 microns or less is used in someembodiments. In certain desirable implementations, a coating of 1 micronor less is used.

IV. Further Applications for Embodiments of the Disclosed Technology

In general, the disclosed particle detector embodiments and methods formanufacturing such particle detectors can be adapted for use in anyapplication where incident charged particles are desirably detected withhigh spatial resolution. Furthermore, although the discussion aboveprimarily concerned the detection of alpha particles, embodiments of thedisclosed technology can be modified to detect other particles (e.g.,beta particles, neutrons or X-rays) that by interaction with atoms inthe optical fibers will produce light. For instance, the doping agentand doping depth can be selected and adjusted as appropriate dependingon the target particle for which the detector is designed. Similarly,the thickness of the coating on the fiber optic face plate is tailoredto the type of radiation being detected.

For example, embodiments of the disclosed technology can be used inneutron scattering systems (e.g., cold neutron scattering experiments,such as those performed at facilities like the Spallation Neutron Source(“SNS”) or the High Flux Isotope Reactor (“HFIR”). An exampleconfiguration for such a system is illustrated in FIG. 9. In particular,FIG. 9 shows a portion of a scattered neutron detection system 900 inwhich partially doped optical fibers 910, 912, 914, 916, 918 are alignedin a spherical arrangement around a scattering origin. The scatteringorigin corresponds to the location of a scattering target 922 upon whichneutrons (e.g., cold neutron 924) are impinging. As illustrated, thepath of the cold neutron 924 extends from a neutron source 920 to thescattering target 922, where it is potentially scattered along a newpath and incident with one of the partially doped optical fibers 910,912, 914, 916, 918. The scattered neutron 924 will then interact withthe scintillation region of the corresponding optical fiber, therebygenerating light (one or more photons) that propagates to an oppositeend of the activated optical fiber. As with the embodiments describedabove with respect to FIGS. 6 and 7, the ends of the optical fibers canbe communicatively coupled to a photomultiplier (e.g., a pixelatedphotomultiplier or position-sensitive photomultiplier) or otherlight-to-electrical-signal converter (e.g., via a light guide or bundleof fibers). The location of the scattered neutron can then be determinedthrough appropriate processing (e.g., implemented usingcomputer-executable instructions executed by a computer and stored on anon-transitory computer-readable medium). Although only four partiallydoped optical fibers 910, 912, 914, 916, 918 are shown, it should beunderstood that additional partially doped optical fibers can beincluded in the system 900 and arranged to provide a suitable detectingsurface that provides positional detection of a scattered neutron withhigh spatial resolution. The partially doped optical fibers 910, 912,914, 916, 918 can be manufactured using any of the techniques discussedabove. However, in order for the optical fibers to scintillate uponimpingement by a neutron, the one or more doping agents and/or dopingdepths can be modified. In certain embodiments, for example, multipledoping agents are used: a first doping agent to interact with anincident neutron and produce a secondary particle, and a second dopingagent to interact with the secondary particles produced and generate oneor more photons. In particular embodiments, the first doping agent islithium (e.g., lithium-6 (⁶Li)) or boron (e.g., boron-10 (¹⁰B)), and thesecond doping agent is cerium (e.g., Ce³⁺), europium (e.g., Eu²⁺) orpraseodymium (e.g., Pr³⁺). In neutron absorption in ⁶Li, for example, atriton and alpha particle are produced. These particles will travel ashort distance in the glass before they are stopped and produce light byscintillating via the second doping agent. The neutron absorption crosssection of ⁶Li at 0.0252 eV is 945 barns. The size of the scintillationregion on a fiber designed to detect neutrons will typically depend onthe absorption cross section, the number of ⁶Li atoms per cubiccentimeter, and the energy of the neutrons. Cold neutrons typically varyin energy from 3×10⁻⁷ to 5×10⁻⁵ eV. For a glass of density of 2.5 g/cm³,with 6.6% Li and with 95% of ⁶Li, about 95% of the neutrons (withenergy=5×10⁻⁵ eV) will be absorbed in 53 microns of material. However,for cold neutron energy of 3×10⁻⁷ eV, the neutron cross section is muchhigher and the thickness to absorb 95% of the neutrons is about 10microns. Thus, in certain embodiments, the doping depths in the opticalfibers is between 10 and 60 microns).

For these larger depths (e.g., of 10 microns or greater, or 50 micronsor greater) ion implantation may not be sufficient to achieve thedesired doping depths. In such situations, a diffusion process can beused to achieve the desired doping depths. For example, one exemplarymethod of introducing activator ions into the end of an optical fibercomprises depositing activator ions (e.g., ⁶Li or ¹⁰B) on the end of oneor more optical fibers and then thermally diffusing the activator ioninto the fiber end. The depth of the resulting scintillation region canbe controlled by using an appropriate combination of thermal diffusiontemperature and time. The activator ion can be deposited on the fiberend in either metallic, oxide, halide or other chemical form. Radialimplantation could also be used. In certain embodiments, this diffusionmethod is used in conjunction with either single or multiple ionimplantations.

The thickness of the coating that covers the scintillation end can alsobe modified for neutron detection applications. For example, the coatingcan be thicker than alpha-particle detection applications becauseneutrons can more easily penetrate the coating. For example, the coatingcan have a thickness greater than 5 microns (e.g., a thickness of 5-100microns). This increased thickness also improves the reflectivity ofphotons within the fibers, making light generation in the fibers moreefficient.

In FIG. 9, the scintillating region is integrally formed within theoptical fibers. This integrated design provides a number of possibleadvantages, any one or more of which can be realized in implementationsof the disclosed technology used for cold neutron scatteringapplications. As above, the one or more possible advantages that can berealized include a simpler design, small spatial light dispersion in thescintillator, concentration of light into single fibers, high spatialresolution for cold neutron detection (e.g., 100 microns or less),improved light output from the neutron reaction at the output side ofthe optical fibers, lower false detection rate, higher real detectionrate, low sensitivity to X-rays, low sensitivity to gamma rays, ornanosecond or less time resolution.

Having illustrated and described the principles of the illustratedembodiments, it will be apparent to those skilled in the art that theembodiments can be modified in arrangement and detail without departingfrom such principles. For example, any of the disclosed system can beused in conjunction with a gamma-ray interrogation system or betaparticle detection system. Additionally, any of the disclosedembodiments can be used with or adapted for use with molecularscattering systems, such as those used to develop clothing, medicalmaterials, electronic materials, or any other new material. In suchsystems, the number of partially doped optical fibers, or size ofdetection systems using such partially doped optical fibers may befairly large. In view of the many possible embodiments to which theprinciples of the disclosed invention may be applied, it should berecognized that the illustrated embodiments are only preferred examplesof the invention and should not be taken as limiting the scope of theinvention. Rather, the scope of the invention is defined by thefollowing claims. We therefore claim as our invention all that comeswithin the scope and spirit of these claims.

1. A particle detection system, comprising: a plurality of opticalfibers having first ends and second ends opposite the first ends, theoptical fibers comprising doped regions at the first ends and non-dopedregions adjacent to the doped regions, the doped regions of the opticalfibers comprising a dopant having a dopant depth, the dopant and dopantdepth being selected to scintillate upon interaction with a singletarget particle type, and to thereby generate one or more photons, theoptical fibers being further arranged to form a detection surface fordetecting a spatial location on the detection surface where the singletarget particle type is detected; and a pixelated photomultipliercommunicatively coupled to the plurality of optical fibers, thepixelated photomultiplier being configured to produce an electricalsignal in response to the one or more photons indicative of the spatiallocation on the detection surface where the single target particle typeis detected.
 2. The particle detection system of claim 1, wherein thephotomultiplier is communicatively coupled to the optical fiber via alight guide or via a direct coupling.
 3. The particle detection systemof claim 1, further comprising a coating deposited over the first end ofthe optical fibers, the coating being configured to block transmissionof one or more untargeted particles.
 4. The particle detection system ofclaim 3, wherein the coating is aluminum and has a thickness of 5microns or less.
 5. The particle detection system of claim 1, wherein adepth of the doped region of the optical fibers is 50 microns or less.6. The particle detection system of claim 1, wherein the doped regioncomprises multiple dopants.
 7. The particle detection system of claim 1,wherein at least one dopant of the doped region of the optical fiber isone of cerium, europium, or praseodymium, and wherein the single targetparticle type is an alpha particle.
 8. The particle detection system ofclaim 7, wherein a depth of the doped region is 20 microns or less. 9.The particle detection system of claim 1, wherein at least one dopant ofthe doped region is one of lithium or boron, and wherein the singletarget particle type is a neutron.
 10. The particle detection system ofclaim 1, wherein a first dopant of the doped region is one of cerium,europium, or praseodymium, and a second dopant of the doped region isone of lithium or boron.
 11. The particle detection system of claim 10,wherein a depth of the doped region is between 20 and 100 microns. 12.The particle detection system of claim 1, wherein the single targetparticle type is an alpha particle, a beta particle, or a neutron. 13.An associated particle imaging system comprising the particle detectionsystem of claim
 1. 14. A neutron detection system comprising theparticle detection system of claim
 1. 15. A beta particle detectionsystem comprising the particle detection system of claim
 1. 16. Theparticle detection system of claim 1, wherein the dopant and dopantdepth are further selected to scintillate upon interaction with aparticle of the single target particle type having a selected particleenergy.
 17. A neutron radiography system, comprising: a neutron source;one or more neutron detectors positioned to detect at least some of theneutrons generated by the neutron source; an interrogation regionlocated between the neutron source and the one or more neutrondetectors; and an alpha particle detector configured to detect alphaparticles associated with neutrons generated by the first neutronsource, the alpha particle detector comprising the particle detectionsystem of claim
 1. 18. The system of claim 17, wherein the doped regionsof the optical fibers of the particle detection system form alphaparticle scintillation regions.
 19. The system of claim 18, wherein amajority of the bodies of the optical fibers are undoped.
 20. The systemof claim 18, wherein the doped regions of the optical fibers are dopedwith one or more of cerium, europium, or praseodymium.
 21. The system ofclaim 17, wherein the alpha particle detector has a spatial resolutionof 100 microns or less.
 22. The system of claim 17, wherein the alphaparticle detector has a timing resolution of 2 nanoseconds or less. 23.The system of claim 17, wherein the alpha particle detector furthercomprises a coating configured to prevent transmission of light andscattered accelerated particles to the doped regions of the opticalfibers.
 24. The system of claim 17, wherein the alpha particle detectordoes not include a scintillating element separable from the opticalfibers.
 25. The system of claim 17, wherein the pixelatedphotomultiplier is communicatively coupled to the optical fibers througha light guide or a direct coupling.
 26. The neutron radiography systemof claim 17, further comprising a computer coupled to the pixelatedphotomultiplier and programmed to compute a direction of travel of aneutron based on the spatial location on the detection surface where anassociated alpha particle is detected.
 27. A neutron scattering system,comprising: a neutron source; a scattering target positioned in a pathalong which neutrons from the neutron source travel; one or more neutrondetectors positioned to detect one or more scattered neutrons scatteredby the scattering target, the one or more neutron detectors comprisingthe particle detection system of claim
 1. 28. The neutron scatteringsystem of claim 27, wherein the doped regions of the optical fibers ofthe particle detection system are doped with two or more dopants, andwherein the two or more dopants comprise a first dopant selected togenerate a secondary particle upon interaction with the incident neutronand a second dopant selected to generate a photon upon interaction withthe secondary particle.
 29. The neutron scattering system of claim 28,wherein the first dopant is one or more of lithium or boron, and thesecond dopant is one or more of cerium, europium, or praseodymium. 30.The system of claim 27, wherein a majority of the bodies of the opticalfibers of the particle detection system are undoped.
 31. The system ofclaim 27, wherein the first ends of the optical fibers of the particledetection system are covered by a coating that permits transmission ofneutrons and reflects photons generated within the bodies of the opticalfibers.